Peel the prawns and set aside; reserve the heads and shells. Heat a large, deep pan with 2 tbsp of the vegetable oil. Dust the rabbit legs with salt and the flour, then brown well all over in the hot oil. Remove and set aside.
Add the remaining oil to the pan, then sweat the onion until soft, without colouring. Add the prawn heads and shells, garlic, fennel seeds, peppercorns, star anise, bay leaves, thyme and bunch of tarragon sprigs. Cook for 5 minutes. Stir in the tomato paste and wine and cook for a further 5 minutes. Add half of the chicken stock and bring to the boil, then add the remaining stock and bring up to the boil again. Return the rabbit legs to the pan. Simmer for 1 hour, until the legs are cooked through.
Meanwhile, make the pastry. Combine the flour, butter and salt in a food processor and blitz until the mix resembles breadcrumbs. Add just enough iced water to bind, blitzing briefly to combine. Wrap and leave to rest for half an hour in the fridge before rolling out.
Remove the rabbit legs from the cooking liquor and allow to cool slightly, then remove the meat from the bone, keeping it in good-size chunks. Set aside.
Pass the sauce through a fine sieve into a bowl, pressing well on all the flavourings (if you have a good blender, blend half of the sauce, then mix it with the rest of the sauce before sieving). Heat the 50g butter in saucepan, when melted add the flour and mix well. Add a little of the cooking liquor, whisk well then add the remaining liquid. Simmer gently until the flour has cooked out then taste and adjust the seasoning, then finish with the chopped parsley and tarragon and the mustards. Add the prawns and rabbit chunks and mix well.
Roll out the pastry on a lightly floured surface to 4mm thick and cut to the required shape to make a lid for your pie dish or dishes. Spoon the filling into the pie dish(es). Brush the rim of the pie dish(es) with egg yolk, then place the pastry lid on top and press to the rim to seal.
Preheat the oven to 180°C/350°F/gas mark 4. Brush the pastry lid with egg yolk then bake for about 20 minutes until golden brown. Allow to rest for 5 minutes before serving.